The present invention relates generally to fabrication of objects and prototypes through the sequential deposition of material. More particularly, the invention relates to the consolidation of material increments using electrical resistance to form objects.
Numerous manufacturing technologies for producing objects by sequentially adding material exist, with the casting of liquid metal being perhaps the oldest such technique. In the past two decades, various processes for fabricating objects to net shape solely through material addition, i.e. without a finishing step such as machining to produce detailed, high-precision features, have been patented and, in a few cases, commercialized.
Most of these additive manufacturing processes either rely on an adhesive, or a solidification process in order to produce a bond between previously deposited material and each incremental volume of material which is added. Although the use of adhesives is convenient, the properties of the adhesive control the properties of the finished object, and this limits the usefulness of such processes in the production of engineering parts and products.
Processes which use solidification transformations result in objects with relatively uniform physical and mechanical properties, because the liquid which is present as each volume of material is added wets the previously deposited material, effectively acting as an adhesive with properties identical to those of the bulk material.
The most commercially successful of these technologies is stereolithography, in which a focused light source (typically an ultraviolet laser) is used to solidify a liquid photocuring polymer. As the laser focal point travels through a vat of liquid polymer, the polymer locally solidifies, and eventually, through appropriate programming of the motion of the focal point, a solid object is built.
Although several techniques have been developed and commercialized, the technologies available for additively producing metal objects are limited. Since the Bronze Age, humans have used forging as a means of producing objects by adding small volumes of material to shapes and hammering them to final dimensions. More recently, three-dimensional arc welding (shape melting), as described and patented by Edmonds et al., (U.S. Pat. No. 4,775,092) has been suggested as an approach to production of net shape metal components.
Prinz, U.S. Pat. No. 5,207,371, has also developed shape deposition modeling in which two types of molten metal are sequentially deposited to produce net shape. Prinz and others have shown that in addition to arc welding, laser deposition and thermal spraying may be used as the basis for forming net shape objects layer by layer, if masks are used at intervals sufficient to define the cross sections of the desired object (See U.S. Pat. No. 5,126,529). Kovacevic has refined the methods of Edmonds and included milling to improve object dimensional accuracy.
Laser melting and deposition have been developed extensively in the U.S. and Germany. Based on cladding technologies developed in the 1980s, processes such as laser engineered net shaping and direct metal deposition are being commercialized (See Lewis, U.S. Pat. No. 5,961,862). Laser direct metal deposition is under development by researchers around the world, including Sandia National Laboratory, Los Alamos National Laboratory, Optomec Inc., and Precision Optical Manufacturing in the United States, and the Fraunhofer Institute in Germany. In essence, the process involves the injection of metal powders into a high power laser beam, while the laser is rastered across a part surface. The powders are melted in the beam, and deposited under the influence primarily of gravity.
Other processes for producing net shape metal objects via material deposition involve the use of low melting point materials to join sheets or powders. For example, brazing of laminated objects has been described (patents) in which steel sheets are cut to the geometries of sequential cross sections of a part, and then furnace brazed together. A copper, titanium or nickel based braze alloy is generally used, with copper alloys having the lowest melting points, and ease of use.
A closely related technique uses infiltration of a low surface tension, low melting point alloy to fill voids in object made by compacting or printing metal powders has also been employed. For example, Cima et al. have patented a three-dimensional printing process, in which metal powders are ink jet printed in layers, and a binder is used to hold the shape of the printed object (U.S. Pat. No. 5,387,380). Following firing of this green part to remove the binder, the infiltrant can be added to produce a solid metal object (Dillon Infiltrated Powdered Metal Composite Article (U.S. Pat. No. 4,327,156). This technique is being commercialized by Extrude Hone Corporation. Other powder metallurgy techniques for producing metal objects to net shape involve the use of a pattern against which powders are densified using various combinations of elevated temperatures and pressures to produce a fully dense, net shape part.
In U.S. Pat. No. 5,578,227, Rabinovich describes a method in which a wire or filament feedstock is used and applied to a growing object while maintaining a substantially identical cross section by remotely heating the nit point at which the feedstock is fed onto and is tangent to the existing surface. Rabinovich proposes use of a laser to heat this location to them melting point.
Electroforming, or plating, has also been commercialized for additive manufacturing of metal components. This mature technology has recently been used to produce shells on near net shape patterns to produce objects, usually tooling inserts for the injection molding process. Electroforming is a very slow process. It typically takes up to two weeks to produce a shell 0.25xe2x80x3 thick in a material such as nickel which has sufficient strength and wear resistance to be used as permanent tooling. As a result, this process is used only to create shells which require backfilling by some secondary material. Metal powder filled epoxies are most often used, however, ceramic slurries, other plastics, cements, and low melting points metals have all been used.
Electroforming has other drawbacks besides extremely low deposition rate as a near net shape forming technology. In the electroforming process, metal salts are dissolved in an aqueous solution. When an electrical current passes through this bath, metal is deposited on the negatively charged surface (in net shape electroforming applications such as tooling, this will be a model which is the inverse of the desired final shape). Aqueous solutions of metal salts are generally toxic, and sludges form in these baths as a byproduct of the process. Both the liquid and the sludges are hazardous materials which must be handled and disposed of appropriately. It is noteworthy that Andre has patented a method of fabricating layered structures using masks and electroplating (U.S. Pat. Nos. 5,976,339 and 5,614,075).
More recently, nickel vapor deposition has been employed as a means of producing nickel shells for net shape fabrication applications. Nickel vapor deposition (NVD) allows thicker shells to be produced as deposition rates are higher than electroforming (Milinkovic, 1995). However, NVD involves the use of highly toxic gases and a specialized reaction chamber. The cost and risk of this technology are both very high.
Resistance heating is a widely used process for fabricating structures as diverse as automobile bodies, electronic equipment and piping. It operates on the principle that heat is generated when electrical current flows through a conducting medium. The amount of heat generated is proportional to the current flow and degree of resistance to it in the carrying medium according to Q=I2Rt, where Q is heat generated in joules, I is current in amperes, R is resistivity of the material in ohms, and t is time in seconds.
In terms of technology, oxygen-free high-conductivity (OFHC) copper has very low resistance, and can carry very high currents with little heating. Nickel-chromium alloys have high resistivity and are used to produce heating elements; when high currents are passed through them, the elements produce heat for ovens, furnaces, water heaters, etc.
In most resistance joining applications, high-conductivity copper electrodes are used to conduct current through the lower conductivity work pieces. The interface between the two workpieces is the location of the greatest resistance in the circuit, and it will heat up the most quickly and to the highest temperature. This is illustrated in FIG. 1. Conveniently, this is the very location at which a joint is desired. The current is selected to provide conditions under which a joint can form, and pressure is applied via the electrodes. Depending on the application, the electrical current and pressure may be pulsed through several cycles. A range of resistance joining processes exist including spot welding, seam welding, stud welding, flash butt welding, etc., but so far, the approach has not been applied to the consolidation of net-shape or near-net shape objects.
Novel processes for additive manufacturing of net shape objects composed of metals are clearly needed. The technologies described above are limited in their capability, use expensive equipment, and typically have safety hazards associated with the presence of lasers, liquid metals and powders.
This invention resides in an additive manufacturing apparatus and methods wherein resistance heating, preferably with applied pressure, is applied uniformly, or cyclically, so as to consolidate incremental volumes of material to produce a desired object in accordance with a description thereof.
Depending upon the particular parameters of the process, the joining of the material increments may occur in the solid state, liquid state or xe2x80x98mushyxe2x80x99 state. In all embodiments, however, the process produces an atomically clean faying surface between the increments without melting the material in bulk.
Residual stresses are minimized, particularly in metal objects, by imposing a compressive residual stress on the surface of each deposited layer or increment, which offsets all or a portion of the tensile stress created as the next layer deposited above it cools. In terms of apparatus, a moving cathode is used to ensure uniform electrical current flow in an object with constantly changing geometry.
In the preferred embodiment, the contact resistance of the interface between the workpieces is continuously measured, and the sensor data is used to update a look-up table, or as input to an adaptive closed loop control system to ensure consistent welding conditions as object geometry changes continually. Also disclosed are embodiments associated with the fabrication of functionally gradient materials.